1. Field of the Invention
This invention relates generally to servo control for data storage devices, and more particularly to repeatable runout (RRO) compensation methods and apparatus for servo-controlled data storage devices.
2. Description of the Related Art
High precision design is required for direct access storage devices (DASDs), such as disk drives. Without limiting operating vibration performance, the servo architecture of a DASD should be capable of providing a low positioning error for its read/write heads. In a track following mode, a disk actuator servo intends to minimize the head position error in the presence of both repeatable runout (RRO) and non-repeatable runout (NRRO) components in a position error signal (PES). The RRO component, which could be any repeated deviation from an ideal circular track due to, for example, mechanical tolerances or servo writing, may present itself as a repetitive disturbance having a fundamental frequency and several harmonics.
There are many conventional methods for minimizing head positioning error by reducing the RRO component. Many of these techniques attempt to reduce the RRO at the manufacturing level by imposing very severe manufacturing tolerance requirements, or by implementing gain enhancing algorithms, such as feedforward or narrow band filters at RRO spectral frequencies. However, these approaches either require high cost manufacturing methods or increased track following voice coil motor (VCM) power.
In FIG. 1, a typical DASD servo has an overall structure which includes a PES generator 100, a servo computation block 102, and a digital-to-analog converter (DAC) 104. The structure also involves a feedforward scheme which includes a misposition corrector 106 for improving track following capability. As shown in FIG. 1, an output of misposition corrector 106 is summed with an output of servo computation block 102 at a summing node 108. The summed output at node 108 is then provided to DAC 104. An analog output of DAC 104 is then provided to a current driver 110 which provides current to the VCM of an actuator 112, which represents the mechanical components that control the position of a transducer which interacts with a data storage medium of the DASD.
The control signal fed into PES generator 100 is initially computed based on a PES signal derived from the data surface. The PES signal is processed by servo computation block 102 and modified by a misposition correction signal (also called a “feedforward” signal) output from misposition corrector 106. The signal into PES generator 100 is the relative difference between a head position signal received from actuator 112 and runout components 114 of a disk track, which invariably contains both RRO and NRRO components. While a subtraction node 116 is shown, this is in fact a theoretical node as it is only the difference generated by PES generator 100 that is available when the recording head signal is processed for use by the servo loop. Misposition corrector 106 is designed to produce a misposition correction signal which cancels or reduces signals of runout components 114.
It has been observed that RRO components vary as a function of disk position, not only radially (i.e. across tracks, e.g. from inner to outer tracks) but circumferentially as well (i.e. across sectors, e.g. from sector to sector along the same track). Some conventional compensation schemes provide for RRO compensation based on disk location. However, the known prior art does not focus on a “global” learning scheme to compensate for RRO. Some of the prior art techniques consider all disk tracks as a whole using an averaging function, but they do not take into account variations of amplitude and phase across the disk. Other techniques focus on learning RRO characteristics of each track, but this requires a large amount of memory. Efficiencies in computation, design, and memory use are all desirable features to have in an RRO compensation scheme.
FIG. 2 is a three-dimensional graph 200 which reveals an RRO harmonic amplitude component 202 with respect to cylinder number (e.g. track number) and sector number of a disk drive. As illustrated, RRO characteristic 202 is shown to vary from sector to sector in a sinusoidal fashion over the disk. As shown, there is a “ramping” of the RRO from the outer diameter (OD) to the inner diameter (ID) of the disk. This particular ramping is due to a periodically distributed clamping force from screws on top of a disk clamp of the disk drive. There is a similar RRO variation due to thermal expansion mismatch between the hub and the disk.
FIG. 3 is a graph 300 which reveals an RRO characteristic 302 with respect to cylinder number and a prior art compensation signal 304 intended to reduce the RRO. In this example, compensation signal 304 is generated based on an average of RRO measurements from track to track. As apparent from FIG. 3, compensation signal 304 is not entirely suitable to accurately reduce RRO characteristic 302. In FIG. 4, a graph 400 which reveals RRO characteristic 302 with respect to cylinder number and another prior art compensation signal 402 intended to reduce the RRO. In this example, compensation signal 402 is generated from a plurality of RRO measurements from disk track to disk track, each measurement of which must be stored. As apparent from FIG. 4, compensation signal 402 is also not entirely suitable to accurately reduce and RRO characteristic 302 and a relatively large amount of memory must be utilized.
What are needed are improved methods and apparatus for RRO compensation for data storage devices.